Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation; full citations for these documents may be found at the end of the specification immediately preceding the claims. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
Gels are a unique class of materials which exhibit solid-like behavior resulting from a continuous three-dimensional framework extending throughout a liquid. This framework consists of molecules interconnected through multifunctional junctions. These junctions can be formed, for example, through covalent crosslinking, crystallization, ionic interactions, hydrogen bonding, or chain entanglements. In some cases, junction formation is reversible and the gels revert to liquid-like behavior upon a change in temperature.
Gels have been synthesized for a variety of applications. For example, gels have been used in the electrophoresis of protein mixtures, as chromatographic packing material, and as a contact lens material. Gels have also been used as intermediates to produce other products. For example, gels have been used in the fabrication of high modulus fibers, membranes, metal oxide ceramics, and low density materials (often referred to as foams, aerogels, and xerogels).
The nomenclature of foams, acrogels, and xerogels is often arbitrary, confusing, and inconsistent. These terms generally pertain porous, lightweight, relatively low density materials.
Generally, the term "foam" is used to refer to low density porous materials which may often be conveniently and simplistically characterized as dispersions of gas bubbles in a material, which material may be liquid or solid. Foams have found widespread utility in a variety of applications. For example, foams formed from organic polymers have found use in insulation, construction, filtration, and related industries.
Foams are often conveniently classified as closed cell foams or open cell foams. Closed cell foams are primarily characterized by having sealed pore volumes from which the entrapped gas cannot easily escape. A common example of a closed cell foam is polystyrene. In contrast, open cell foams are primarily characterized by having pore volumes which are not sealed, and are often interconnected, and from which entrapped gas can escape or re-enter.
Several classes of open cell foams have been somewhat arbitrarily identified. The term "aerogel" is often used to identify one class of open cells foams characterized by transparent, low density, high surface area porous solids composed of interconnected colloidal-like particles or fibrous chains. These materials are characterized by morphological structures (e.g., particle sizes and spacings, pores size and spacings) having dimensions which are less than about 100 nm. Consequently, these materials are visibly transparent.
Aerogels are also characterized by the nature of their porosity (e.g., pore size distribution): typically, they possess micro-pores, meso-pores, and macro-pores. Micro-pores generally include pores with dimensions less than about 2 nm; these are often pores within or between individual particles. Meso-pores are generally pores with dimensions about 2 to about 50 nm; these are often associated with the spacing between particles or chains of particles. In aerogels, macro-pores are generally pores with dimensions about 50 nm to about the 100 nm. The high surface areas of aerogels (e.g., 300 to 1000 m.sup.2 /g) is attributed to the porous nanostructure.
The term "xerogel" is often used to identify another class of open cells foams which are, in many respects, similar to aerogels (e.g., lightweight, porous, high surface area, and transparent), but are notably more dense than aerogels. In this way, a xerogel is often described as a densified version of an aerogel; that is, wherein the material has contracted to bring particles and particle chains closer together. Thus, xerogels are characterized by a reduction in the number of macro- and meso-pores, often ascribed to the compaction of particles and the removal of voids between particles and chains of particles. The observed surface area for xerogels (often as high as 500 m.sup.2 /g) is primarily a result of a large number of micro-pores within and between individual particles.
Another class of open cells foams, often referred to simply as "foams," are porous solids characterized by being opaque and having low density and relatively high surface area. These materials are characterized by some morphological structures (e.g., particle sizes and spacings, pores size and spacings) having dimensions which are generally larger than 100 nm. Consequently, these materials are visibly opaque.
An important class of organic foams may be derived from the gels formed by the polycondensation of a substituted (e.g., hydroxylated, often polyhydroxylated) aromatic compounds with an aldehyde. Like other chemically-linked gels, the small cell/pore size of the resulting organic gel (typically less than about 50 nm) necessitates complex, intensive, and expensive drying methods to yield an organic aerogel. Large capillary forces at the liquid-vapor interface cause the gel to shrink or crack if the solvent is removed by simple evaporation yielding substantially densified and undesired products.
To overcome this limitation, new methods to effect drying have been developed. By using supercritical extraction of the pore fluid, e.g., by evaporation, apparently no surface tension is exerted across the cells/pores, and the dried aerogel retains much of the original morphology of the initial gel. See, for example, Pekala, 1989a. In this method, the water in the pores of the water-containing gel is exchanged with acetone to form an acetone-containing gel; the acetone in the acetone-containing gel is then exchanged with liquid carbon dioxide which is then removed under supercritical conditions (e.g., CO.sub.2 critical temperature=31.degree. C., critical pressure=7.4 mPa) to produce the dried organic aerogel. Alternatively, certain organic gels can often be dried, for example, by solvent exchange. See, for example, Mayer et al., 1995a. In this method, the water in the pores of the water-containing gel is exchanged with acetone to form an acetone-containing gel; the acetone in the acetone-containing gel is then exchanged with cyclohexane; and the cyclohexane in the pores of the cyclohexane-containing gel is then removed by simple evaporation to produce the dried organic aerogel.
An important class of organic gels which have been the focus of efforts to produce aerogels and xerogels are the hydroxylated benzene-aldehyde gels; that is, gels obtained by the polycondensation of hydroxylated benzene compounds, such as phenol, resorcinol, catechol, hydroquinone, and phloroglucinol, with aldehydes, such as formaldehyde, glyoxal, glutaraldehyde, and furfural. Particularly common examples are the resorcinol-formaldehyde (i.e., RF) gels. See, for example, Pekala, 1989a, 1989b, 1991, and 1992.
Resorcinol, also referred to as 1,3-dihydroxybenzene (i.e., C.sub.6 H.sub.4 (OH).sub.2), undergoes most of the typical reactions of phenol (i.e., C.sub.6 H.sub.5 OH), but at a much faster rate because of the enhanced electron density in the 2-, 4-, and 6-ring positions. Resorcinol, like phenol, is known to react with formaldehyde (i.e., CH.sub.2 O) under alkaline conditions to form mixtures of addition and condensation products. The principle reactions involved include: (1) the formation of hydroxymethyl (--CH.sub.2 OH) derivatives of resorcinol and (2) condensation of the hydroxymethyl derivatives to form methylene (--CH.sub.2 --) and methylene ether (--CH.sub.2 OCH.sub.2 --) bridged compounds.
A resorcinol-formaldehyde polymer (so-called RF polymer) may be formed in which resorcinol monomers are linked by methylene and methylene ether bridges to form a chemically crosslinked network.
A simple example is illustrated below. ##STR1##
The resorcinol-formaldehyde reaction, often referred to as the RF reaction, is usually performed in aqueous solution. Resorcinol, a white solid, is usually dissolved in a an aqueous formaldehyde solution (37.6 wt % aqueous formaldehyde is widely commercially available), and a suitable catalyst, such as sodium carbonate (Na.sub.2 CO.sub.3), often as a dilute aqueous solution, is added. In some cases, an acid catalyst, such as trifluoroacetic acid, is employed. The solution is gently warmed to initiate the gelation reaction, and to form a chemically cross-linked RF gel. If the gel is properly formulated, it is often possible to dry the gel to yield an RF aerogel, but only using complex, intensive, and expensive methods such as supercritical extraction, subcritical evaporation, and solvent exchange methods.
Carbon foams, and more particularly carbon foams having densities of less than about 100 mg/cm.sup.3 and cell sizes of less than about 25 microns, have been prepared using a variety of methods, including pyrolysis of an organic polymer aerogel under an inert atmosphere such as N.sub.2 or argon. See, for example, Kong, 1991a, 1991b, and 1993. Owing primarily to their high electrical conductivity, carbon foams have found wide utility in electrode applications such as energy storage devices (e.g., capacitors and batteries), fuel cells, and electrocapacitive deionization devices. See, for example, Pekala et al., 1995a; Mayer et al., 1994, 1995b, 1995c, 1996, 1997; and Kashmitter et al., 1993 1996. Carbon foams have also found utility in variety of other applications, including filtration media, catalyst supports, and structural media.
Similarly, once obtained, the dried RF aerogel may often be converted to a carbon aerogel by pyrolysis under an inert atmosphere such as N.sub.2 or argon. Such carbon aerogels are electrically conducting and are particularly useful as electrodes in double layer capacitors for energy storage or for capacitive deionization (see, for example, Pekala et al., 1995b). Carbon aerogels which are useful in capacitance applications typically possess a porous carbon matrix comprised of interconnected carbon particles and are characterized primarily by a large number of micro-pores and meso-pores, found within and between the particles.
Since the carbon aerogel is derived from the RF organic aerogel (via pyrolysis), it is expected that the structure of the RF aerogel will impact the properties of the carbon aerogel. In principle, the particle size and pore structure of the carbon aerogel is patterned after the microstructure of the RF aerogel.
A critical parameter impacting the morphology of the RF aerogel is the value of the reaction parameter "R/C", discussed in more detail below, is the molar concentration of resorcinol or its functional equivalent! divided by the molar concentration of catalyst!. At high catalyst concentrations (e.g., R/C.about.50), very small RF particles (e.g., .about.9 nm) are formed, whereas at low catalyst concentrations (e.g., R/C.about.900), large RF particles (e.g., .about.65 nm) are formed. Also, at high catalyst concentrations (e.g., R/C.about.50), high surface areas (e.g., 900 m.sup.2 /g) are found, whereas at low catalyst concentrations (e.g., R/C.about.300), lower surface areas (e.g., .about.390 m.sup.2 /g) are found. The high surface areas found with high catalyst concentrations may be explained by the formation of a large number of very small particles. At higher catalyst concentrations (e.g., R/C&lt;300-400), the RF gels are transparent and homogenous and result in dried RF aerogels that are dark red in color but still transparent to visible light. At lower catalyst concentrations (e.g., R/C&gt;300-400), the organic gels are typically opaque and result in dried organic aerogels which are not transparent to visible light (due to the formation of large particles and pore spaces which scatter visible light). Since they are not transparent, the latter materials must have particle and/or pore sites larger than about 100 nm. and, as a result, are often no longer considered to be aerogels.
For RF aerogels prepared at low catalyst concentration (high R/C), it has been observed that the surface area of a carbon aerogel was greater than the surface area of the RF aerogel from which it was obtained (see, for example, Pekala et al., 1982). It has been suggested that this increase in surface area is due to a decrease in particle size and the formation of additional pores as volatile by-products are released during pyrolysis. This trend is reversed for RF aerogels prepared at high catalyst concentration (low R/C) because the particles are already so small that they, instead, tend to fuse during carbonization leading to a loss of surface area.
It has been suggested that only the surface of the particles in the meso-pore region (2 to 50 nm) are responsible for formation of the electrical double layer that gives rise to the energy storage characteristics of the carbon aerogel (see, for example, Mayer et al., 1993). In this way, materials which have few or no meso-pores, such as xerogels, have very low electrical capacitance, whereas materials with substantial numbers of meso-pores often have substantial electrical capacitance. Carbon aerogels derived (via pyrolysis) from RF aerogels having a solids content of about 40% w/v and greater, possess mesopore distribution of .ltoreq.7 nm (Tran et al). The capacitance of RF-derived carbon aerogels has been observed to peak at densities of 500 mg/cm.sup.3 and higher.
In the known methods for forming aerogels from hydroxylated benzenes (such as resorcinol) and aldehydes (such as formaldehyde), reaction parameters such as pH and solids content, are carefully controlled so that a useful product (e.g., dried aerogel) may be obtained from the gel. That is, reaction parameters are carefully controlled so that the resulting gel can be successfully dried to form an aerogel which retains much of the morphology of the original gel.
For example, it has been reported that RF gels formed using low catalyst concentrations (high R/C, initial room temperature pH below 6.0) yield materials which, upon drying, form opaque materials characterized by large particles and large pore/cell size, and generally not considered to be aerogels. It has thus been generally believed that the RF gels formed using unconventional reaction parameters are not useful in the formation of RF aerogels and their related carbon foams, primarily because such gels either have an undesirable morphology or are thought to be unable to retain their morphology upon drying. Much of the effort in the field of low density open cell foams has been directed to aerogels and xerogels, which are invariably derived from transparent gels. Consequently, there has been little interest in the preparation and/or drying of gels formed under reaction conditions that typically lead to opaque gels, and little expectation that such opaque gels would yield foams with many of the properties of aerogels.
In conventional methods, the pH range of the reaction mixture, which is primarily determined by the concentration of catalyst, is carefully controlled to yield a reaction mixture pH which falls in the narrow range of 6.5 to 7.4. Typically, the reaction parameter R/C, which is the ratio of the number of moles of hydroxylated benzene compound to the number of moles of catalyst and which thus helps determine the pH of the reaction mixture, is selected to be from about 50 to about 400. Also in conventional methods, the solids content is carefully controlled. Typically, the reaction parameter R, which is the weight % of the hydroxylated benzene compounds and aldehydes in the reaction mixture with respect to total volume, is selected to be from about 5-40% w/v.
Using these conventional methods and reaction parameters, gels are obtained which may be successfully dried (albeit using complex, intensive, and expensive methods) to produce organic aerogels and, upon pyrolysis, carbon aerogels, both of which possess typical aerogel properties, such as transparency (except for carbon aerogels), high surface area, ultrafine particle size, and porosity. However, these conventional gels cannot survive simple evaporative drying of high surface tension solvents such as the water pore fluid. Upon simple evaporative drying, the gels shrink and crack. The resulting products do not retain the morphology of the original gel.
We have found, inter alia, that by moving outside of the conventional parameters, and more particularly, by moving to lower catalyst concentrations (i.e., an R/C value of greater than about 1000, yielding an initial room temperature pH of lower than about 6.0), new organic gels are obtained which, upon curing, yield strong organic gels. These new organic gels, unlike the gels produced using conventional reaction parameters, are sturdy enough to withstand simple (and cheap) evaporative drying of the solvent water contained in the pores--perhaps due to the large size cells and pore spaces--without the need for complex, intensive, and expensive drying methods such as supercritical evaporation, subcritical evaporation, or solvent exchange methods. The resulting unique low density open cell organic foams have many of the useful properties associated with aerogels. In this way, new low density open cell organic foams are obtained which are characterized by relatively large particle and pore sizes, high porosity, and high surface area. These organic aerogels may be pyrolyzed to form new low density open cell carbon foams also characterized by relatively large particle and pore sizes, high porosity, high surface area, and further characterized by high capacitance.